1. Field of the Invention
The present invention relates to a water-disintegratable sheet which can be used for cleaning article, toilet paper, topsheet or backsheet of absorbent article, packaging sheet for wrapping absorbent article or the like, and a method for manufacturing the same.
2. Description of the Related Art
Wet sheets for wiping discharging parts (e.g., the anus) of the body and toilet papers to be used in dry condition are preferred to be disintegratable (decomposable) in water. In absorbent articles such as sanitary napkin, panty liner, disposable diaper and the like, topsheet for covering the top face of absorbent layer and backsheet for covering the back face of the absorbent layer are preferred to be disintegratable in water. Moreover, packaging sheets for wrapping such absorbent articles are also preferred to be disintegratable in water.
If water-disintegratable sheets are used in such products, they can be disposed of in flush toilet after use When the water-disintegratable sheet is disposed of in flush toilet, a large amount of water is given thereto in the flush toilet or in a septic tank. Therefore, constituent fibers of the water-disintegratable sheet are dispersed in water, thereby preventing the sheet from floating to stay in the septic tank.
Water-disintegratable sheets of this kind are required to maintain a certain strength in use and allow their constituent fibers to disperse when a large amount of water is given thereto.
In order to impart such characteristics, conventional water-disintegratable sheets are typically constructed such that a fiber structure in the form of nonwoven fabric is given a binder for bonding the fibers, such as a water-soluble or water-swellable carboxyl methylcellulose or a water-soluble polyvinyl alcohol. In this water-disintegratable sheet, the sheet strength is obtained by the binder in use, and when a large amount of water is given thereto, the binder is dissolved or swollen to disconnect the fibers.
On the other hand, Japanese Unexamined Patent Publication No. 11-206611 (206611/1999) discloses water-disintegratable tissue paper comprising water-dispersible fibers and microfibrillated cellulose. This water-disintegratable tissue paper is manufactured by blending the water-dispersible fibers and the microfibrillated cellulose in a wet-laid process, followed by drying. In this water-disintegratable sheet, the bonding strength between the water-dispersible fibers can be obtained through a hydrogen bonding power of the microfibrillated cellulose, and when a large amount of water is given thereto, the hydrogen bonding power is reduced to disconnect the water-dispersible fibers.
However, the sheet containing the water-soluble or water-swellable binder needs an additional step of applying such binder, making the production process complicated. In addition, it is not desirable that the sheet containing such binder is brought into direct contact with the skin of the human body. Especially a water-disintegratable sheet to be used in wet condition such as wet tissue is formed such that the foregoing water-disintegratable sheet is impregnated with a liquid containing electrolyte for suppressing dissolving or swelling of the binder in wet condition. However, such electrolyte is undesirable because it may be irritating to the skin.
In the sheet disclosed in Japanese Unexamined Patent Publication No. 11-206611, on the other hand, the water-dispersible fibers are bonded through the strong hydrogen bonding power of the microfibrillated cellulose, and the density of the sheet is increased because of the microfibrillated cellulose present between the water-dispersible fibers. Therefore, the stiffness of the sheet is excessively high in dry condition and the sheet surface is hard. Accordingly, when used as toilet paper, the sheet gives hard feeling to the user""s body.
Moreover, when the tissue paper disclosed in the publication is impregnated with a liquid, the hydrogen bonding is weakened to extremely lower the bonding power between the water-dispersible fibers. Therefore, the tissue paper cannot be used in wet condition because of its weak sheet strength.
The present invention has been worked out in view of the shortcoming in the prior art set forth above. It is therefore an object of the present invention to provide a water-disintegratable sheet which can reduce stiffness and provide softness and in which strength and water-disintegratability can be easily balanced, and a method for manufacturing the same.
According to a first aspect of the present invention, there is provided a water-disintegratable sheet comprising water-dispersible fibers and microfibrillated cellulose, the water-dispersible fibers being hydroentangled about each other to provide high fiber density regions and low fiber density regions, the hydroentangled water-dispersible fibers being bonded to each other through a hydrogen bonding power of the microfibrillated cellulose.
In the water-disintegratable sheet of the present invention, since the high fiber density regions and the low fiber density regions are repeatedly formed by a water-jet treatment, although the water-dispersible fibers are firmly bonded through the hydrogen bonding power of the microfibrillated cellulose, the stiffness of the sheet can be reduced and the sheet can be softened. Moreover, since the sheet strength is obtained by both the hydrogen bonding power of the microfibrillated cellulose and the entanglement with the water-jet treatment, the sheet strength can be maintained even when used in wet condition.
Preferably, the water-disintegratable sheet contains 70 to 95% by weight of water-dispersible fibers and 5 to 30% by weight of microfibrillated cellulose. In this case, the sheet strength and the water-disintegratability can be easily balanced in both dry and wet conditions.
Preferably, the microfibrillated cellulose has a mean fiber length of 0.3 to 1.5 mm and a mean fiber diameter of 0.001 to 0.1 xcexcm. In this case, the microfibrillated cellulose has a large surface area. Also preferably, the microfibrillated cellulose has a viscosity of 1,000 to 10,000 mPaxc2x7s, where 2% by weight of microfibrillated cellulose is mixed with 98% by weight of water. In this case, the microfibrillated cellulose has a dense network structure similar to the cellulose molecule, thereby exhibiting a strong hydrogen bonding power due to an OH group on the surface thereof. Therefore, the water-dispersible fibers can be firmly bonded to increase the sheet strength.
Preferably, the sheet has an average density equal to or less than 0.3 g/cm3. In this case, the stiffness of the sheet can be reduced and the sheet can be made soft. On the other hand, the lower limit of the average density is preferably 0.05 g/cm3.
Preferably, the microfibrillated cellulose is present more in the high fiber density regions than in the low fiber density regions. In the water-disintegratable sheet of the invention, the water-dispersible fibers are gathered and entangled mainly in the high fiber density regions. In the case where the microfibrillated cellulose is collected in these high fiber density regions, the water-dispersible fibers can be sufficiently bonded to each other through the hydrogen bonding power of the microfibrillated cellulose even if the water-dispersible fibers are loosely entangled. Therefore, the sheet strength can be maintained high.
Preferably, the water-dispersible fibers have a fiber length equal to or less than 10 mm and equal to or more than 3 mm. If the fiber length exceeds 10 mm, the water-dispersible fibers will be excessively entangled about each other by water jets, thereby making it difficult for the water-dispersible fibers to be disentangled in water. If the fiber length is below 3 mm, on the other hand, the strength due to entanglement of the water-dispersible fibers can not be expected.
Preferably, the water-dispersible fibers are biodegradable fibers. In this case, the fibers can be biodegraded after dispersion in water, preventing environmental pollution.
Preferably, the square root of the product of the tensile strength in MD and the tensile strength in CD is from 2 to 4 N for 25 mm width, where the water-disintegratable sheet is impregnated with distilled water, which weighs twice as heavy as the sheet. Also preferably, the square root of the product of the tensile strength in MD and the tensile strength in CD is from 4 to 13 N for 25 mm width, where the water-disintegratable sheet is in dry condition. By setting the sheet strength within such ranges, when used as a cleaning article, the sheet can endure a frictional force imparted in wiping operation. On the other hand, when used for an absorbent article, the sheet can maintain the entire shape of the product.
According to a second aspect of the present invention, there is provided a method for manufacturing a water-disintegratable sheet comprising:
blending water-dispersible fibers and microfibrillated cellulose in a wet-laid process to obtain a fibrous web containing the microfibrillated cellulose in an amount of 5 to 30% by weight;
applying water jets to the fibrous web to hydroentangle the water-dispersible fibers about each other and to provide low fiber density regions to which the water jets are applied and high fiber density regions to which fibers removed from the low fiber density regions by the water jets are gathered; and
drying the fibrous web to bond the hydroentangled water-dispersible fibers to each other through a hydrogen bonding power of the microfibrillated cellulose.
According to the water-disintegratable sheet manufacturing method of the present invention, the sheet which is soft, strong and easily disintegratable in water can be obtained by using wet-laid process and water-jet treatment which are both widely used in the art.
Preferably, the microfibrillated cellulose has a mean fiber length of 0.3 to 1.5 mm and a mean fiber diameter of 0.001 to 0.1 xcexcm. Also preferably, the microfibrillated cellulose has a viscosity of 1,000 to 10,000 mPaxc2x7s, where 2% by weight of microfibrillated cellulose is mixed with 98% by weight of water.
Preferably, a processing energy of each water jet treatment imparted to the fibrous web with a single row of water-jet nozzles arranged in CD is from 0.05 to 0.5 kw/m2 and the water-jet treatment is performed 1 to 6 times.
By setting the processing energy of the water jet treatment within such ranges, the water-dispersible fibers can be appropriately entangled, maintaining the sheet strength high during use and facilitating disentanglement of the water-dispersible fibers when a large amount of water is given.